The present invention relates to a method of forming a multicolor image and, more particularly, to a multicolor image forming method for executing, after a toner image has been formed on an image carrier by a sequence of charging, exposing and developing steps, another sequence of charging, exposing and developing steps using a toner different in tone from the toner of the toner image existing on the image carrier to thereby form toner images of at least two colors on a single image carrier one above the other. The present invention is also concerned with an apparatus for practicing such a method.
Image forming methods capable of forming multicolor images, e.g., bicolor images are disclosed in, for example, Japanese Patent Laid-Open Publication Nos. 23952/1982, 116553/1983, and 121349/1084. Basically, the conventional bicolor image forming methods commonly involve a first charging step which uniformly charges the surface of an image carrier to a potential of negative polarity, a first exposing step for exposing the charged surface of the image carrier by image data associated with a toner image of first color to form a first electrostatic latent image, a first developing step for developing the first latent image by a negatively charged toner of first color under the application of a bias and by reversal development so as to form a toner image of first color, a second charging (recharging) step for recharging the image carrier carrying the toner image of first color to a uniform potential, a second exposing step for exposing the recharged surface of the image carrier to image data associated with a toner of second color to form a second electrostatic latent image, and a second developing step for developing the second latent image by a toner of second color and of the same polarity as the image carrier under the application of a bias and by reversal development.
After the sequence of steps stated above, the bicolor toner image is transferred to a paper or similar recording medium and then fixed. Usually, the second developing step is implemented by a non-contact developing system to prevent the toner image of first color from being disturbed, i.e., prevent the different colors from being mixed and prevent the toner image from being disfigured.
Japanese Patent Laid-Open Publication No. 17464/1985 teaches a bicolor image forming method which increases the surface potential of a toner layer developed a first electrostatic latent image to an anti-color mixture potential close to the initial surface potential. Specifically, this method applies an AC voltage with at least a predetermined DC voltage component thereof made offset in the event of the second charging step, thereby increasing the potential of the image portion of the image carrier.
Japanese Patent Laid-Open Publication No. 127082/1991 proposes an implementation for preventing the toner of second color from being mixed with the toner of first color by maintaining the charge potential of the second charging step higher than that of the first charging step at all times.
Further, Japanese Patent Publication Nos. 45916/1989 and 22947/1990 disclose methods which make the potential of the image portion of the image carrier substantially equal to the potential of the non-image portion in the second charging (recharging) step.
However, the problem with the conventional methods is that as the iterative image forming steps, i.e., the first charging step to the second developing steps are performed over a long period of time, the toner image of second color becomes impure and this degrades the image quality as a whole. This stems from the fact that during the non-contact second developing step the toner image of first color flies reversely from the image carrier to the developing unit storing the toner of second color. This is also true with a multicolor image consisting of toner images of three or more colors. Further, when the charge potential of the second charging step is higher than that of the first charging step, the ripple of potential occurred on the surface of the image carrier during the first charging step is superposed on the ripple of potential occurring during the second charging step. As a result, the ripple width is apt to increase. In addition, increasing the charge potential brings about various problems in respect of power consumption, service life of the image carrier, ozone ascribable to the high voltage, etc. Therefore, the charge potential for the second charging step should preferably be equal to or lower than that for the first charging step.